The present invention relates to superconducting materials, and more specifically, to textured superconducting materials with high critical temperature and high critical current density.
The phenomenon of superconductivity was first discovered in the early 1900s. A superconducting material conducts current with zero energy loss and expels magnetic field (like a perfect diamagnetic material) when cooled below the transition temperature. Until the mid 1980s, all known superconducting materials were metallic compounds such as mercury (Hg), lead (Pb), and niobium-tin (Nb3Sn). In general, these materials become superconducting at temperatures below 40 degrees Kelvin, depending on the material, by undergoing a transition from the normal, resistive state to the superconducting state. The transition temperature (Tc) is a material specific temperature. For any material in the superconducting state at a given temperature and applied magnetic field, there is a maximum current density that the material is able to conduct without developing resistance. The critical current density (JC) is also one of the factors that limits the maximum magnetic field Hc at which a superconductor can remain in the superconducting state. As the externally applied magnetic field (H) increases, the critical current density JC(T, H) decreases. Above some critical field, Hc, the material can not support any current in the superconducting state and undergoes transition to the normal state. Both Hc(T) and JC(T) increase when decreasing the cooling temperature of the superconductor.
Depending on certain magnetization properties, a superconducting material can be characterized as a type I superconductor or a type II superconductor. When increasing the applied current or magnetic field, or raising the temperature above Tc, type I superconductors undergo a direct transition from the perfectly diamagnetic state (i.e., the Meissner state) to the normal state. Type II superconductors, however, first develop a xe2x80x9cmixed (vortex) state,xe2x80x9d wherein the applied magnetic field penetrates the superconducting material above the lower critical field (Hc1), and then the material undergoes the transition to the normal state above the upper critical field (Hc2). When the magnetic field is raised above Hc1, it becomes energetically more favorable to admit into the material individual flux quanta in vortices than to maintain the Meissner state with the total flux exclusion. The vortices are distributed over the superconducting material to achieve an energetic minimum. When a transport current passes through the superconductor in the mixed state, the Lorentz force acts on the vortices. At the same time, chemical and physical defects in the superconducting material may keep the vortices xe2x80x9cpinnedxe2x80x9d at the location of the defect. If the Lorentz force (which is proportional to the current density) exceeds the pinning forces, the vortices start to move and dissipate heat, which leads to resistivity.
In the mid 1980s, first high temperature superconductors (HTS) based on oxides of copper compounds were discovered. Some of these materials displayed superconductivity above liquid nitrogen temperature (Tc greater than 77K) allowing dramatically more practical and economical cooling. For example, the HTS materials are compounds of RE1Ba2Cu3O7xe2x88x92xcex4 wherein RE=Y, Nd, La, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu; the Bi2Sr2CaCu2Ox, (Bi, Pb)2Sr2CaCu2Ox, Bi2Sr2Ca2Cu3Ox and (Bi, Pb)2Sr2Ca2Cu3Ox compounds; the Tl2Ca1.5BaCu2Ox or Tl2Ca2Ba2 Cu3Ox compounds and compounds involving substitution such as the Nd1+xBa2xe2x88x92xCu3Ox compounds. These copper oxide superconductors are type II superconductors.
Several researchers have focussed on introducing controlled defects into the HTS materials to increase the pinning forces. These defects reduce the movement of the fluxoids and permit high critical currents even at relatively high temperatures and high magnetic fields. Magnetic field that penetrates the superconducting material may also lead to xe2x80x9ctrappedxe2x80x9d magnetic field. The trapped field can be pinned in place even when there is no supporting external magnetic field. An ingot of superconducting material with trapped magnetic field, not supported by another magnet, is called a trapped field magnet, and is similar in some ways to a permanent magnet.
When the externally applied magnetic field (or the applied current) is removed, the trapped magnetic fields decay over time, which is called flux creep. Flux creep tends to stabilize the flux distribution in the superconducting material by relieving the magnetic pressure. Flux creep, which decays approximately logarithmically over time, is a measure of loss of the trapped magnetic field. The magnetic flux density is supported by the pinning force and is related to the current density by Ampere""s law. As the current density is increased toward the critical current density JC(T) flux creep increases.
As mentioned above, particular defects increase pinning of the fluxoids. The optimal defect diameter is determined by two parameters of the superconducting material. The first parameter is the magnetic field penetration depth, which determines how far from the defect the magnetic field may penetrate into the superconducting material. The second parameter is the coherence length, which determines how far into the superconductor, from the defect, the vortex current builds up.
In 1989, vanDover et al. reported that JC and Hirr improve when YBa2Cu3O7 single-crystals are irradiated either with thermal or fast neutrons (See R. B. van Dover et al., xe2x80x9cCritical Currents Near 106 A/cm2 in Neutron-irradiated Single-Crystal YBa2Cu3O7xe2x80x9d, Nature, vol. 342, pp. 55-57, 1989.).
Civali et al. reported creation of columnar defects in YBa2Cu3O7 crystals by 580-MeV Sn-ion irradiation. The columnar defects allow the field to pass through the material and also serve as pinning centers with attractive potentials that reduce the flux creep. (See Civali et al., xe2x80x9cVortex Confinement by Columnar Defects in Yba2Cu3O7 Crystals: Enhanced Pinning at High Fields and Temperaturesxe2x80x9d, Phys. Rev. Left., vol. 67, p. 648, 1991).
Weinstein et al. reported increase in the critical current density of the YBCO material exposed to high-energy light-ion irradiation. (See R. Weinstein et al., xe2x80x9cMaterials, Characterization and Applications for High TC Superconducting Permanent Magnetsxe2x80x9d, Applied Superconductivity, Vol. 1, pg. 1145-1155, Pergammon Press, 1993).
Fleischer et al. reported some increase in JC and Hirr, in polycrystalline bulk HTS materials or films doped with a mixture of uranium-238 and uranium-235 and subsequently irradiated with thermal neutrons to cause fission. (See Fleischer et al., xe2x80x9cIncreased Flux Pinning upon Thermal-Neutron Irradiation of Uranium-doped Yba2Cu3O7xe2x80x9d, Phys. Rev. B, Vol. 40, pp. 2163-2169, 1989; Luborsky et al., xe2x80x9cCritical Currents after Thermal Neutron Irradiation of Uranium Doped Superconductorsxe2x80x9d, J. Mater. Res., Vol. 6, pp. 28-35, 1991; Fleischer et al., xe2x80x9cIncreased Flux Pining Upon Thermal-Neutron Irradiation of Uranium-Doped Yba2Cu3O7xe2x80x9d, Gen. Electric Tech. Report #89CRD047, April 1989; and U.S. Pat. No. 4,996,192)
Safar et al. reported improved JC and Hirr in the HTS bismuth materials irradiated with high-energy protons (800 MeV). (See Appi. Phys. Lett. vol. 67, p. 130, 1995)
In general, there is a need for a high Tc superconducting material with Tc above 77 K and high values of JC and Hirr, which can be economically produced in uniform bulk quantities, or in form of thick or thin films, and which are suitable for different superconducting applications.
In one aspect, the invention is a textured high Tc superconducting material that includes a chemical element or compound that can be fissioned. The fissionable element or compound does not spoil the texturing. The textured high Tc superconducting material may be subjected to neutron irradiation to further increase JC and Hirr of the material. The textured, irradiated superconducting material may be economically produced in uniform bulk quantities, or in form of thick or thin films, and does not have high residual radioactivity. While neutron irradiation is preferred, the texturing of the HTS material is essential.
Texturing, as defined herein, includes any process of aligning microcrystals or growing larger crystals in a bulk sample, and also includes xe2x80x9cnaturalxe2x80x9d texturing that occurs when a thick film is deposited (for example, by spin coating) or a thin film is deposited by any of the known physical deposition method (for example, sputtering, evaporation, epitaxial growth) and in processed in-situ or ex-situ. Without texturing the polycrystalline HTS has very low intergrain current density. The fissionable element or compound may be uranium-235 or an intimate mixture of uranium-235 and uranium-238, or plutonium-239, which is added to a precursor superconducting material in an additional step before texturing the material.
In another aspect, the invention offers an irradiation enhanced, textured oxide superconducting material with significantly higher JC and Hirr than superconducting materials currently made with other techniques. Bulk samples of the textured oxide superconducting material having 2 cm in diameter and 0.8 cm in length achieve average current densities of 85,000 A/cm2 and trapped magnetic fields above 20,000 Gauss. This superconductivity material in the size of a 3 mm cube achieves average current densities of 300,000 A/cm2 at 2,500 Gauss at T77K, and achieves 106A/cm2 at 2,500 Gauss at T=50k.
In another aspect, an oxide superconductor includes a textured superconducting material including an array of defects finely dispersed throughout the superconducting material. These defects include a compound that comprises barium, oxygen, at least one rare earth element, and at least one of the following chemical elements: uranium-238, Nd, Mn, Re, Th, Sm, V, and Ta.
Preferred embodiments of these aspects include one or more of the following features: The one rare earth element is preferably yttrium, neodymium or samarium, but may also be lanthanum, europium, gadolinium, dysprosium, hafnium, erbium, thulium, ytterbium or lutetium or their combination. The compound forms a pinning site. If the rare earth element is yttrium and the chemical element is uranium, the compound has substantially the following atomic ratio (U0.4Y0.6)BaO3; alternatively, yttrium and uranium are replaced with any of the elements listed in the two groups, but such compound still has a substantially similar stoichiometry of as the uraniumxe2x80x94yttrium compound. The compound may include platinum. If platinum is included, the rare earth element is yttrium and the chemical element is uranium, the compound has substantially the following atomic ratio (U0.6Pt0.4)YBa2O6; alternatively, yttrium and uranium are replaced with any of the elements listed in the two groups, but such compound still has a substantially similar stoichiometry as the uranium-platinum-yttrium compound.
In another aspect, an oxide superconductor includes a textured superconducting material including an array of defects with a neutron-fissionable element. The array of defects is dispersed throughout the superconducting material.
Preferred embodiments of this aspect include one or more of the following features: The defects may be predominantly between 1 nanometer and 1000 nanometers in size, but preferably the defects are between 1 and 100 nanometers. The oxide superconductor may further include a matrix of dispersed randomly oriented defects from 1 to 1000 nanometers in diameter and 0.1 to 20 micrometers in length created by fission. The defects may predominantly be individually aligned sets of broken columnar defects, bead-shaped defects or irregularly shaped defects. Such defect sets measure predominantly from 1 to 1000 nanometers in diameter and 0.1 to 20 micrometers in length, but preferably between 3 to 100 nanometers in diameter and less than 20 micrometers in length.
The oxide superconductor may include a finely dispersed matrix of fission products that are aligned with the columnar or aligned defects. The neutron-fissionable element may be provided in the amount of 0.001% to 6% of the weight of the material. The neutron-fissionable element may be uranium or plutonium. The neutron-fissionable element may be uranium-235 in the amount of 2 parts per million by weight to 4% by weight of the superconducting material.
In another aspect, an oxide superconductor includes a textured superconducting material with an array of defects including a compound of a neutron-fissionable element, wherein the array of defects is dispersed throughout the superconducting material.
In another aspect, an oxide superconductor includes a textured superconducting material with an array of defects including a compound of a neutron-fissionable element and platinum, wherein the array of defects is dispersed throughout the superconducting material.
Preferred embodiments of the above aspects include one or more of the following features: The textured superconducting material is a BiSrCaCuO superconducting material, a (Bi, Pb)SrCaCuO superconducting material, a YBaCuO superconducting material or a TlBaCaCuO superconducting material. The neutron-fissionable element is uranium and the compound further includes barium, oxygen and at least one rare earth element. The rare earth element is preferably yttrium, neodymium or samarium, but may also be lanthanum, europium, gadolinium, dysprosium, hafnium, erbium, thulium, ytterbium or lutetium or their combination.
Preferred embodiments of the above aspects may also include one or more of the following features: The oxide superconductor, wherein the neutron-fissionable element is uranium and the compound further includes barium, oxygen, and at least one rare earth element. The compound includes chemical elements with substantially the following atomic ratios (U0.6Pt0.4)YBa2O6 or (U0.4Y0.6)BaO3. The defects formed by the compound are predominantly between 1 nanometer and 1000 nanometers in size. The compound includes uranium (or a chemical element with substitutional properties such as Nd, Sm, Mn, Re, V or Ta) in an amount of 0.01% to 6% of the weight of the material. The neutron-fissionable element is uranium-235 in an amount of 2 parts per million by weight to 4% by weight.
The oxide superconductor includes a matrix of finely dispersed randomly oriented defects created by fission. The randomly oriented defects have columnar shape with 1 to 1000 nanometers in diameter and 0.1 to 20 micrometers in length. The randomly oriented defects are finely dispersed and randomly oriented and have the form of aligned broken columns, beads and irregular shapes with 1 to 1000 nanometers in diameter and 0.1 to 20 micrometers in length. The oxide superconductor includes a finely dispersed matrix of fission products, the products spatially aligned with the columnar or broken columnar defects.
Preferred embodiments of the above aspects may also include one or more of the following features: A superconducting trapped-field magnet made of the above-described oxide superconductors capable of maintaining a persistent circulating current within the oxide superconductor material. The current density of the circulating current is in the range 100 to 10,000,000 amps per square centimeter. A magnetic shield made of the above-described oxide superconductors capable of maintaining in a superconducting state a persistent circulating current within the oxide superconductor material. A superconducting wire made of the above-described oxide superconductors. A magnetic levitator made of the above-described oxide superconductors. The magnetic exhibits in a superconducting state the current density in the range 100 to 10,000,000 amps per square centimeter.
In another aspect, an oxide superconductor is made by a method including the steps of: providing a precursor superconducting material; providing a neutron-fissionable element; combining the precursor superconducting material with the neutron-fissionable element; and texturing the precursor superconducting material combined with the neutron-fissionable element to form a textured superconducting material including an array of defects with a neutron-fissionable element, the array of defects being dispersed throughout the superconducting material.
In another aspect, an oxide superconductor is made by a method including the steps of: providing a precursor superconducting material; providing at least one of the following chemical elements: uranium-238, Nd, Mn, Re, Th, Sm, V, and Ta; combining the precursor superconducting material with at least one of the chemical elements; and texturing the precursor superconducting material combined with the chemical element to form a textured superconducting material including an array of defects with the chemical element, the array of defects being dispersed throughout said superconducting material.
Preferred embodiments of the above aspects include one or more of the following features: The oxide superconductor is one of the following: a BiSrCaCuO superconducting material, a (Bi, Pb)SrCaCuO superconducting material, a YBaCuO superconducting material, and a TlBaCaCuO superconducting material. The precursor superconducting material is a stoichiometric mixture or a non-stoichiometric mixture of chemical elements of an oxide superconductor. The chemical element is included in a compound, and the non-stoichiometric mixture includes a selected deviation from a stoichiometry of the oxide superconductor, the deviation being selected with respect to stoichiometry of the compound including the neutron-fissionable element, or another compound suitable for formation of pinning sites. The neutron-fissionable element is included in a compound, and the non-stoichiometric mixture includes a selected deviation from a stoichiometry of the oxide superconductor, the deviation being selected with respect to stoichiometry of the compound including the amount of the neutron-fissionable element and any other chemical element substituted for the neutron-fissionable element. The neutron-fissionable element substitutionally replaces one or more elements of the above superconducting materials.
Preferred embodiments of the above aspects may also include one or more of the following features: The neutron-fissionable element is in form of a powdered oxide and the combining step of the method includes mixing the powdered oxide with the precursor superconducting material. The neutron-fissionable element in an amount of 0.01% to 6% by weight of the final material. The neutron-fissionable element includes uranium-235 in an amount between 2 parts per million and 4% of the oxide superconductor by weight. The step of providing the neutron-fissionable element includes: producing a solution of the neutron-fissionable element by dissolving the element in an acid; neutralizing the acid; and precipitating the neutron-fissionable element from the solution as an oxide. The acid includes nitric acid. The acid is neutralized with ammonium hydroxide. The method further includes the step of irradiating with neutrons the textured precursor superconducting material combined with the neutron-fissionable element. The irradiating step includes exposing the textured precursor superconducting material combined with the neutron-fissionable element to the total neutron fluence between 1014 and 1019 neutrons per square centimeter.